1. Field of the Invention
The present invention relates to the field of fabricating photomasks for use in the manufacture of semiconductor devices and, more particularly, to the fabrication of phase shifting photomasks having improved transmission characteristics for use in sub-micron lithography techniques.
2. Related Applications
This application is related to co-pending application entitled, "Phase Shifting Mask Structure with Absorbing/Attenuating Sidewalls for Improved Imaging", Ser. No. 007,638, filed Jan. 21, 1993; and copending application entitled, "Method of Fabricating Phase Shifters with Absorbing/Attenuating Sidewalls Using An Additive Process", Ser. No. 007,640, filed Jan. 21, 1993.
3. Prior Art
Various techniques are known in the prior art for manufacturing devices on a semiconductor wafer, such as a silicon wafer. Typically, lithography processes are utilized to overlay a pattern(s) onto the wafer. Each pattern provides for selected portions of the wafer to undergo a particular lithographic process, such as deposition, etch, implant, etc. Photomasks (masks) are generally utilized to overlay a particular pattern on the wafer or a layer formed on the wafer. Generally a number of these masks are required for manufacturing a complete device on the wafer.
The earlier prior art lithography techniques rely upon optical techniques in which light is passed through a mask to overlay a pattern on the wafer. Generally, a pattern on the mask equated to a pattern design appearing on the surface of the wafer. However, as the semiconductor technology evolved to allow ever smaller device structures to be fabricated on a wafer, it became increasingly difficult to continue to use the standard optical techniques. It is generally theorized that as device features approach submicron dimensions of 0.25 microns and below, alternative techniques would be required to project patterns onto a wafer.
Due to the limitation imposed by the wavelength of light, resolution at the edges of these patterns tend to degrade when ordinary optical techniques are employed. Standard optical techniques utilizing ultra violet (UV) light will extend the lower range, but still fall short of desired resolution at extremely low ranges (under 0.25 microns). It was generally believed that technologies employing shorter wavelength would ultimately be required for lithography. A number of approaches have been suggested with x-ray lithography being viewed as the technology for use at these low submicron ranges.
However, recent experimentation in the area of phase shifting masks (PSMs) have shown that the PSM technology can be employed to extend the range of optical techniques currently being utilized. That is, the current I-line (at a wavelength of 356 nanometers) and deep ultra violet, or DUV (at a wavelength of 248 nanometers), optical photolithography techniques can be used with the phase shifting photomasks to provide the requisite resolution with sufficient depth of focus for fabricating semiconductor devices having dimensions in the order of 0.25 microns and below. It is believed that resolutions in the order of 0.1 micron resolution levels can be obtained with sufficient focus latitude by the use of ordinary lithography techniques when phase shifting techniques are applied.
It is generally understood that the technique for improving resolution in photolithography by the use of phase-shifting masks was first proposed by Levenson et al., ("Improving Resolution in Photolithography with a Phase-Shifting Mask", IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, December 1992, pp. 1828-1836) and later implemented by Terasawa et al. ("0.3-micron optical lithography using a phase-shifting mask", Proceedings of SPIE, Vol. 1088 Optical/Laser Microlithography II, 1989, pp. 25-32.
The conventional PSM comprises of creating phase shifting regions in the transparent areas of a photomask. These phase-shifting regions are formed either by depositing transparent films of appropriate thickness and then patterning them over the desired transparent areas using a second level lithography and etch technique or by etching vertical trenches into the quartz substrate.
The etched depth or shifter film thickness is designed to produce the desired 180 degree phase shift at the proper incident wavelength (for example, I-line or DUV). In both cases, the interface between the quartz and the surrounding medium (typically air), as well as the edges or walls between the phase shifted and unshifted regions, usually occurs as a sharp transition between high and low refractive index regions. This sharp transition in index from quartz to air causes significant reflections and "scattering" of light in undesired directions and this effect causes an overall loss in transmitted intensity through the mask. In addition, the unequal light intensities from the phase shifted and unshifted regions cause variations in pattern dimensional control (such as to critical dimensions or minimum features), due to an undesired aerial image from the mask.
Furthermore, in addition to the edge effects, the index mismatch between quartz and air causes backward reflections into the mask substrate over the entire area, leading to an overall loss in transmission intensity to the target and a loss in overall intensity and contrast.
It is appreciated that an improved PSM that addresses the edge scattering of light, as well as the backward reflections, would improve image and exposure characteristics.